Support for the standard, unprefixed version of {{ cssxref("calc") }} has been landed. (bug 771678)

The {{ xref_cssresolution() }} CSS data type has been extended to support the dppx. (bug 741644)

On screen, for media queries, dppx, dpi, and dpcm are now representing values based on CSS pixels and no more with the physical units. (bug 771390)

Three new pseudo-classes :-moz-meter-optimum, :-moz-meter-sub-optimum, and :-moz-meter-sub-sub-optimum have been added for accessing/styling a {{HTMLElement("meter")}} element in a paraticular state. (bug 660238)

The {{cssxref("-moz-appearance")}} property gains two new values: meterbar and meterchunk. They represent components inside the {{HTMLElement("meter")}} element. (bug 659999)

DOM

Two new properties width and height have been added to the {{ domxref("HTMLInputElement") }} interface. (bug 683855)

IndexedDB properties and methods have been unprefixed, since IndexedDB has reached Candidate Recommendation status. (bug 726378)

The {{domxref("Keyboard")}} interface, still prefixed as mozKeyboard, now has the {{domxref("Keyboard.setSelectedOption()")}} and {{domxref("Keyboard.setValue()")}} methods, as well as the {{domxref("Keyboard.onfocuschange")}} property.

The Window.java and Window.packages attributes have been removed. These were never documented, and hopefully you're not using them!

The CSSRule.type associated with {{ domxref("CSSNamespaceRule") }} has been updated from UNKNOWN_RULE (0) to NAMESPACE_RULE (10). (bug 765590)

JavaScript

WebGL

SVG

MathML

The lspace and rspace attributes of {{ MathMLElement("mo") }} now correctly default to thickmathspace.

Network

Developer tools

There's now a handy developer toolbar you can access by going to Tools > Web Developer > Developer Toolbar, or by pressing Ctrl-Shift-V (Cmd-Opt-V on Mac OS X). This toolbar offers a command line interface as well as buttons for quickly accessing useful tools. The graphical command line interface (GCLI) is easy to expand and additional commands are expected in the future. Type "help" to get a list of supported commands.

The Web Console now displays an error count so you can quickly see how much work you have ahead of you.